This invention relates to the rapid loading and unloading of steam turbine-generators in accordance with the calculated ratio of steam flows under two types of steam admission in a manner to minimize the thermal stresses in order to reduce the turbine load changing time.
Startup and loading of a large steam turbine-generator has become more involved in recent years, as the trend toward larger units results in higher thermal stresses for any given temperature transient. Two factors contribute to thermal stresses during start up. Initially, a mismatch exists between the temperature of the admitted steam and the metal temperature and the degree of mis-match depends upon the past operating history, i.e., whether or not the turbine is involved in a cold start or a hot start. The mis-match is essentially corrected during the acceleration phase of the startup.
Secondly, when the turbine-generator is producing load and steam flow is high enough so that any substantial mis-match cannot exist, the metal temperature will follow steam temperatures closely. Control of metal temperatures and therefore thermal stresses is based primarily on analytical and statistical correlation between stress levels and expected rotor life.
Traditionally, charts and graphs have been provided to allow the operator to reduce the mis-match at a safe rate during the acceleration phase of the startup and to determine allowable rates of change of metal temperature during the loading procedure. Various techniques have been employed to speed up the process of loading the turbine, including heat soaking periods on "turning gear" to reduce the initial mis-match. Initial operation in the less efficient "full-arc" steam admission mode has been used to achieve uniform warming of the high pressure turbine inlet parts.
There have been suggestions in the published prior art of starting up steam turbines using various techniques such as acceleration control, load control, etc. in an effort to minimize startup time without damaging the turbine. These systems are usually predicated on ideal steam generator conditions. Since turbine startups can take several hours, systems which will reduce these times, as well as allow for fluctuations in steam temperature and pressure from the steam generator, are of great value.
A sophisticated approach to startup and loading control by means of continuously calculating rotor surface and bore stresses from speed and temperature measurements, and then loading at a maximum permissible stress are described in U.S. Pat. No. 3,446,224 issued on May 27, 1969 U.S. Pat. No. 3,561,216 issued on Feb. 9, 1971, U.S. Pat. No. 3,588,265 issued on June 28, 1971, and U.S. Pat. No. 3,928,972 issued on Dec. 30, 1975 etc.. Although these patents are useful for achieving rapid startup and loading, from the standpoint of the delay time involved in the generation of thermal stresses, the above teachings are not always satisfactory because in effect the turbine is essentially controlled while monitoring the thermal stress produced in the turbine rotor.